X-RAY IMAGING
Prof. Yasser Mostafa Kadah – www.k-space.orgEE 472 – F2017
Recommended Textbook
Stewart C. Bushong, Radiologic Science for Technologists:
Physics, Biology, and Protection, 10th ed., Mosby, 2012.
(ISBN 978-0323081351)
X-Ray Production
X-Ray Production
Bremsstrahlung x-rays are produced when a projectile electron
is slowed by the nuclear field of a target atom nucleus
In the diagnostic range, most x-rays are bremsstrahlung x-rays
Characteristic x-rays are emitted when an outer-shell electron
fills an inner-shell void
This type of x-radiation is called characteristic because it is
characteristic of the target element
Only the K-characteristic x-rays of tungsten are useful for imaging
Approximately 99% of the kinetic energy of projectile
electrons is converted to heat (Anode heat)
Quantity and Quality of X-ray Beam
General shape of an emission spectrum is always the same, but
its relative position along the energy axis can change
The farther to the right a spectrum is, the higher the effective energy or
quality of the x-ray beam
The larger the area under the curve, the higher is the x-ray intensity or
quantity
Effect of mA and mAs
A change in mA or mAs results in a proportional change in the
amplitude of the x-ray emission spectrum at all energies.
Effect of kVp
As kVp is raised, area under curve increases by approximating
the square of the factor by which kVp was increased
Accordingly, x-ray quantity increases with the square of this factor
Change in kVp affects both amplitude and position of x-ray
emission spectrum
In diagnostic range, 15% increase in kVp is equivalent to doubling mAs
Effect of Added Filtration
Adding filtration to the useful x-ray beam reduces x-ray beam
intensity while increasing the average energy
The result of added filtration is an increase in the average energy of the
x-ray beam with an accompanying reduction in x-ray quantity
Effect of Target Material
The atomic number of the target affects both the number
(quantity) and the effective energy (quality) of x-rays
As the atomic number of the target material increases, the efficiency of
the production of bremsstrahlung radiation increases, and high-energy
x-rays increase in number to a greater extent than low-energy x-rays.
Effect of Voltage Waveform
There are five voltage waveforms: half-wave–rectified, full-
wave–rectified, three-phase/six-pulse, three-phase/12-pulse,
and high-frequency waveforms
Both quantity and quality decrease by ripple
Because of reduced ripple, operation with three-phase power or high
frequency is equivalent to an approximate 12% Increase in kVp, or
almost a doubling of mAs over single phase power.
Factors Affecting X-Ray Quantity
Factors Affecting X-Ray Quality
Half-Value Layer (HVL)
In radiography, quality of x-rays is measured by the HVL
Diagnostic x-ray usually has HVL 3 to 5 mm Al or 3 to 6 cm of soft tissue
Although x-rays are attenuated exponentially, high-energy x-
rays are more penetrating than low-energy x-rays
100-keV x-rays are attenuated at rate of 3%/cm of soft tissue
10-keV x-rays are attenuated at 15%/cm of soft tissue
X-Ray Interaction with Matter
Coherent scattering (energy < 10 keV)
Compton scattering
Photoelectric effect
Pair production (energy > 1.02 MeV)
Important in making an x-ray image
Compton (Incoherent) Scattering
Photoelectric Effect
Photoelectric Effect
Differential Absorption
X-Ray Exponential Attenuation
The total reduction in the number of x-
rays remaining in an x-ray beam after
penetration through a given thickness of
tissue is called attenuation
When broad beam of x-rays is incident on
any tissue, some x-rays are absorbed, and
some are scattered
The result is a reduced number of x-rays, a
condition referred to as x-ray attenuation
Radiologic Units
Air Kerma (Kinetic Energy Released in Matter) (Gya)
Kinetic energy transferred from photons to electrons during ionization
and excitation measured in J/kg where 1 J/kg = 1 gray (Gya)
Absorbed Dose (Gyt )
Radiation energy absorbed in tissue per unit mass with units of J/kg or
Gyt (gray) which depends on tissue type
Sievert (Sv): quantity of radiation received by radiation
workers and populations
Becquerel (Bq): quantity of radioactive material, not the
radiation emitted by that material
Radioactivity and the becquerel have nothing to do with x-rays
Radiologic Units
X-Ray Tube
External structures
Support structure
Protective housing
Glass or metal enclosure. The internal
Internal structures
Anode and cathode
X-Ray Tube Support Structure
X-ray tube and housing assembly are quite heavy
Require support mechanism so radiologic technologist can position them
Mainly ceiling, floor or C-arm support systems
Protective Housing
When x-rays are produced, they are emitted isotropically
That is, with equal intensity in all directions
Only x-rays emitted through window are called useful beam
X-rays that escape through protective housing: leakage radiation
Leakage radiation contributes nothing to diagnostic information and
result in unnecessary exposure of patient and radiologic technologist
Protective housing guards against excessive radiation exposure
and electric shock
Also mechanically protects x-ray tube
Metal or Glass Enclosure
X-ray tube is an electronic vacuum tube with components
contained within a glass or metal enclosure
vacuum allows for more efficient x-ray production and longer tube life
As glass enclosure tube ages, some tungsten vaporizes and
coats the inside of glass enclosure
Alter electrical properties of the tube, allowing tube current to stray and
interact with the glass enclosure resulting in arcing and tube failure
Most common cause of tube failure
Metal enclosures maintain constant electric potential between
electrons of tube current and enclosure
Longer life and less likely to fail
Virtually all high-capacity x-ray tubes now use metal enclosures
Cathode
Cathode is the negative side of the x-ray tube
It has two primary parts, a filament and a focusing cup
Dual-filament cathode allows two focal spots (e.g., 0.5 and 1.5 mm)
Focusing cup is a metal shroud that surrounds filament
Tube current is adjusted by controlling filament current
Anode
Anode is the positive side of the x-ray tube
Two types: stationary (dental) and rotating (general purpose)
Higher tube currents and shorter exposure times are possible with
rotating anode because of their better heat dissipation
Three functions in an x-ray tube:
Electrical conductor that receives electrons emitted by cathode and
conducts them through the tube to the connecting cables and back to the
high-voltage generator
Mechanical support for the target
Thermal dissipation
Target
The target is area of anode struck by electrons from cathode
Focal Spot
Focal spot is the area of target from which x-rays are emitted
The smaller the focal spot, the better the spatial resolution of the image
Unfortunately, as the size of focal spot decreases, heating of target is
concentrated onto a smaller area (limiting factor to focal spot size
Line-focus principle: angling target makes effective area of the
target much smaller than actual area of electron interaction
Radiographic Image Quality
Definition: fidelity with which anatomical structure being
examined is rendered on radiograph
Spatial resolution: ability to image small objects
Contrast resolution: ability to distinguish anatomical structures
Radiographic noise: random fluctuation in intensity of image
Film graininess, structure mottle, quantum mottle, and scatter radiation
Geometric Factors: Magnification
Geometric Factors: Distortion
Unequal magnification of different portions of the same object
is called shape distortion
Distortion depends on object thickness, position, and shape
Thick objects are more distorted than thin objects
If object plane and image plane are not parallel, distortion occurs
Geometric Factors: Focal-Spot Blur
Focal-spot blur is caused by effective size of focal spot
The most important factor for determining spatial resolution
Smaller on anode side than cathode side of the image (Heel effect)
Subject Factors
kVp is the most important influence on subject contrast
Control of Scatter Radiation
Reduced image contrast results from scattered x-rays
Restricting x-ray beam (collimation) reduces scattering
Beam Restricting Devices
Collimation reduces patient radiation dose and improves contrast resolution
Radiographic Grids
Effective device for reducing level of scatter radiation that
reaches image receptor
The principal function of a grid is to improve image contrast
Radiographic Grids
High-ratio and high-frequency grids increase patient radiation dose
When grid is used, radiographic technique must be increased to produce
same image receptor signal by a factor called Bucky (Grid) factor (B)
As Bucky factor increases, radiographic technique and patient dose increases
The higher the grid ratio, the higher is the Bucky factor
The Bucky factor increases with increasing kVp
Radiographic Grids
Grid Cutoff: undesirable absorption of primary x-rays by grid
Greater Attenuation of primary x-rays near edges of image receptor
Radiographic Grids Types
Parallel, Crossed and Focused
Moving Grid (Bucky): reciprocating and oscillating
(-) Require a bulky mechanism that is subject to failure
(-) Distance between patient and the image receptor is increased
(-) Moving grids can introduce motion into cassette-holding device
Advantages of moving grids far outweigh disadvantages
Computed Radiography (CR)
Filmless radiology using special imaging plates
Photostimulable luminescence (PSL)
Computed Radiography (CR)
Computed
Radiography
Screen-Film
Radiography
Proper radiographic
technique and exposure
are essential
Radiographic
technique is not so
critical
Digital Fluoroscopy (DF)
Fluoroscopy: real-time dynamic viewing of anatomic structures
Advantages of DF include the speed of image acquisition and
postprocessing to enhance image contrast
Interventional Radiology
Performing surgical procedures under guidance from
radiographic equipment
Digital Mammography
Radiographic examination of the breast
Digital Mammography spatial resolution limited by pixel size
Superior contrast resolution principally because of postprocessing
Operating Console
Allows radiologist to control x-ray tube current and voltage so
that useful x-ray beam is of proper quantity and quality
Radiation quantity refers to number of x-rays or intensity of x-ray beam
Radiation quality refers to penetrability of x-ray beam and is expressed
in kilovolt peak (kVp) or, more precisely, half-value layer (HVL)
Autotransformer
Power supplied to x-ray imaging system is delivered first to
autotransformer where it provides controlled but variable
voltage to high-voltage transformer
It is much safer and easier to control a low voltage and then increase it
than to increase a low voltage to the kilovolt level and then control its
magnitude
Adjustment of Kilovolt Peak (kVp)
kVp determines the quality of the x-ray beam
Appropriate autotransformer connections can be selected with
an adjustment knob, a push button, or a touch screen
This low voltage from autotransformer becomes the input to high-voltage
step-up transformer that increases voltage to chosen kilovolt peak
Note: kVp meter placed
across output terminals
of autotransformer actually
reads voltage, not kVp.
It registers kilovolts because
of the known multiplication
factor of high voltage
transformer
Control of Milliamperage (mA)
The x-ray tube current, crossing from cathode to anode, is
measured in milliamperes (mA)
Number of electrons emitted by filament is determined by filament
temperature (controlled in turn by filament current)
Thermionic emission is the release of electrons from a heated filament
Space Charge Effect: As the kVp
is raised, anode becomes more
attractive to electrons that
would not have enough energy
to leave the filament. Hence,
this effectively increases mA
with kVp and hence should be
corrected for by special circuit
Exposure Timer
Most exposure timers are electronic, controlled by
microprocessor
Allow wide range of time intervals to be selected and are accurate to
intervals as small as 1 ms
Special kind of electronic timer, called an mAs timer, monitors
product of mA and exposure time and terminates exposure
when desired mAs value is reached
Because the mAs timer must monitor actual tube current, it is located on
the secondary side of the high-voltage transformer
Automatic Exposure Control (AEC)
AEC is a device that measures quantity of radiation that
reaches image receptor and automatically terminates
exposure when image receptor has received required
radiation intensity
High-Voltage Generator
Function: increases output voltage from autotransformer to the
kVp necessary for x-ray production
High-voltage generator contains three primary parts: high-
voltage transformer, filament transformer, and rectifiers
Note: Although some heat is
generated in the high-voltage
section and is conducted to oil,
the oil is used primarily for
electrical insulation
High-Voltage Transformer
High voltage transformer is a step-up transformer
Turns ratio of is usually between 500:1 and 1000:1
High-Voltage Rectification
Rectification is the process of converting AC to DC
Rectification is accomplished with diodes
Transformers operate AC while x-ray tubes need DC
X-rays are produced by acceleration of electrons from cathode to
anode and cannot be produced by electrons flowing in reverse
Single-Phase vs. Three-Phase
Three-phase power is a more efficient way to produce x-rays
than is single-phase power
With three-phase power, voltage applied across the x-ray tube is nearly
constant, never dropping to zero during exposure.
High-Frequency Generator
High-frequency generators produce nearly constant potential
voltage waveform, improving image quality
Rectified power at 60 Hz is inverted to a higher frequency,
from 500 to 25,000 Hz, then transformed to high voltage
Advantage: much smaller size than 60-Hz high-voltage generators
Voltage Ripple Comparison
Less voltage ripple results in greater radiation quantity and
quality
Power Rating
Transformers and high-voltage generators usually are
identified by their power rating in kilowatts (kW)
Power (W) = Current (A) × Potential (V)
For specifying high-voltage generators, the industry standard
is to use the maximum tube current (mA) possible at 100 kVp
for an exposure of 100 ms
This generally results in the maximum available power
Use RMS voltage factor to account for voltage ripples
0.7 of peak in single phase generators
Close enough to 1 in three-phase and high-frequency generators
X-Ray Circuit
Cardinal Principles for Radiation Protection
Simplified rules designed to ensure safety in radiation areas
for occupational workers
Cardinal Principles for Radiation Protection
Minimize Time
Dose is directly related to duration of radiation exposure
Exposure = Exposure rate × Exposure time
Maximize Distance
As distance between source of radiation and person increases, radiation exposure decreases rapidly by inverse square law
If distance from source exceeds 5 times source diameter, it can be treated as point source (assume true and apply inverse square law)
Use Shielding
Positioning shielding between radiation source and exposed persons greatly reduces level of radiation exposure
Shielding used in diagnostic radiology usually consists of lead, although conventional building materials also are used
Shielding
Estimate dose reduction using half-value layer (HVL) or tenth-
value layer (TVL) of barrier material (1 TVL = 3.3 HVL)
Protective apparel
Protective aprons usually contain 0.5 mm Pb (2 HVL – reduction to 25%).
Actual measurements show reduction to approximately 10%
Effective Dose
Effective dose is the equivalent whole-body dose
When only part of body is exposed, as in medical x-ray imaging, risk is
proportional to effective dose (E)
Equivalent whole-body dose is the weighted average of the radiation
dose to various organs and tissues
Patient and Occupational Effective Dose
Covered Material and Suggest
Problems
Chapters 1, 5, 6, 7, 8, 9, 10, 11, 35 of textbook
Attempt questions at the end of each chapter